1. Field of the Invention
The present invention relates to a method apparatus of image processing wherein an image, such as a copier image, including at least one half-tone portion is reproduced by bi-level quantization.
2. Description of the Prior Art
When an image is reproduced with bi-level quantization of black and white, a half-tone image will also be reproduced as a bi-level image. The resolution and the tone (or the reproduction of concentration) are important factors in the image processing. The resolution is important especially for line images such as a character, a symbol and the like, whereas the tone is important for half-tone images such as a picture of a face. Because the resolution and the tone counteract to each other, various kinds of image processing methods in apparatus have been developed in order to make both factors compatible. The compatibility is especially needed for an image wherein both a line image and a tone image are mixed.
The systematic dither processing is effective for reproduction of a half-tone image. In this processing, the concentration signals of an image sampled by a scan of picture elements of N.times.M matrix are divided into P.times.P submatrices, and the concentration signals of each submatrix are binarized with predetermined threshold values of P.times.P matrix of a dither pattern (see, for example, FIG. 1).
An example of image processing of the systematic dither processing will be explained below: First, assume that such a line image 1 as shown in FIG. 2 is detected by sixteen picture elements which constitute a 4.times.4 submatrix. Each of the numerical values given to each of the elements of a submatrix shown in FIG. 3 represents the concentration of the line image at each corresponding picture elements of the submatrix shown in FIG. 2. When a matrix of the Bayer type shown in FIG. 1 is employed as a dither pattern, the numerical value of each element shown in FIG. 3 is compared with that of the counter element of the Bayer type dither matrix which gives a threshold level of each picture element. The element in the submatrix where the concentration is larger than the threshold is binarized as black. FIG. 4 shows a bi-level image thus reproduced.
In a second example, such a half-tone image 2 as shown in FIG. 5 schematically is processed similarly. Each of the numerical values given to each of the elements of a submatrix shown in FIG. 6 represents the concentration of the corresponding picture element of the submatrix shown in FIG. 5. According to the image processing by using a dither matrix of the Bayer type shown in FIG. 1, the half-tone image 2 of FIG. 5 is reproduced as a bi-level image as is shown in FIG. 7.
As mentioned above, in the systematic dither processing, the concentration signal of each picture element in a matrix of N(/mm).times.M(/mm) picture elements are processed in the unit of P.times.P submatrix. If P increases, N/P and M/P decreases, and the resolution becomes lower. On the contrary, if P decreases or if the submatrix becomes small, the tone is deteriorated. Thus, it is difficult to reproduce a half-tone image having a satisfactory quality in respect to both resolution and tone.
For example, an image including a half-tone portion is sampled with picture elements of 8(/mm).times.8(/mm) matrix, and the image is reproduced with black and white picture elements of 8(/mm).times.8(/mm) matrix as a result of the dither processing. If a dither pattern of 2.times.2 matrix is used, the size of each picture element of the image to be reproduced does not become large so that the resolution is not significantly lowered largely while the image cannot be reproduced to provide a tone of high quality. On the contrary, if a dither pattern of 4.times.4 matrix is used, although the tone is improved, the resolution o the image is lowered so that the quality of the reproduced image becomes worse.
In the JPN. Pat. laid open publication No. 138969/1980, a process is disclosed wherein the concentration level of a picture element under consideration is estimated by taking those of the element pictures surrounding the picture element into account in order to determine the bi-level of each surrounding picture elements. This processing method corresponds substantially to a dither processing with a finer dither pattern. Accordingly, it can not essentially improve the resolution of a line image.